Counter-Counter Optical Device (U)

ABSTRACT

Two Fabry-Perot interference filters are used as tandum polarizers for a  h-power laser beam counter measure in an optical scene. The scene radiation is directed on one polarizer; one plane of polarization of the laser beam is transmitted thereby and the other scene radiation is reflected to the other polarizer. The other polarizer transmits the other plane of polarization of the laser beam and reflects the other scene radiation to a photodetector, such as an image intensifier, infrared imager, television camera tube, or a human eye. The laser beam transmitted by the polarizers is trapped by absorbers and cannot harm the photodetector.

The invention described herein may be manufactured, used, and licensedby the U.S. Government for governmental purposes without the payment ofany royalties thereon.

BACKGROUND OF THE INVENTION

This invention is in the field of optical viewers or detectors such asI² (image intensification) and IR (infrared). In particular, it isconcerned with providing a device which is able to counter one of thecounter-measures used against such viewers. This counter-measure is thehigh-power laser; such a laser can do temporary or permanent damage to asensitive optical device (or to the human eye, for that matter). Variousschemes have been made and proposed for dealing with lasercounter-measures; these schemes include fast-acting electro-opticalshutters, absorbing filters, and interference filters. Also, variousbirefringent materials are used, taking advantage of theiracusto-optical or electro-optical properties. Unfortunately, all ofthese schemes have one disadvantage which makes them unsuitable for useas counter-counter measures against high-power lasers; although some ofthe schemes can theoretically block 100% of high-power laser energy in ashort enough time to prevent damage to sensitive optical devices, theirperformances in practice fall short of 100%. Moreover, most such schemestemporarily block, on an optical device optical path, all radiation froma scene (both desired radiation and counter-measure radiation). Thedevice is thus temporarily "blinded" during counter measures. Theinstant invention transmits 100% of counter-measure radiation from anoptical scene while reflecting the at least part of remainder of thescene radiation to a detector. Thus, the invention allows observation ofa scene during counter means; the counter-measure source (laser) willappear as a dark spot in a scene observed by a positive imaging viewer.

SUMMARY OF THE INVENTION

This invention is an optical device usable in the presence of opticalcounter measures such as high-power lasers operating in the responseband of the viewer. The device includes a pair of interference filtersused as tandem polarizers for counter-measure laser radiation from ascene. One filter transmits one plane of polarization of thecounter-measure radiation and reflects a portion of the remainder of thescene radiation. The reflected radiation is directed onto the otherpolarizer, whose plane of polarization is rotated 90° about the deviceoptical axis. This other polarizer transmits the other plane ofpolarization of the reflected counter-measure radiation and reflects aportion of the scene radiation onto an optical detector. This detectormay be I², IR, or television camera tube, or the human eye.

BRIEF DESCRIPTION OF THE DRAWINGS

The single drawing figure is a schematic isometric representation of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENT

The invention might be best understood when this description is taken inconjunction with the drawing. In the drawing, numeral 10 designates anoptical scene employing a counter-measure laser or other high-poweroptical source. The normal radiation from the scene is indicated byexemplary rays 11 and 12. Additionally, countermeasure laser ray 13originates in the scene; although laser radiations are usually planepolarized, it is assumed that ray 13 has radiation in both ordinary andextraordinary planes of polarization. On this assumption, no matter whatthe polarizating orientation of ray 13, it will be diverted by theinvention. Rays 11, 12, and 13 are collimated by lens 14 and fall onplate 15. This plate is a thin-film Fabry-Perot interference filter forray 13, i.e., partially reflective films separated by a distancedependent on the wavelength of ray 13 such that the filter is a goodtransmitter for 13. The filter is not used in its normal mode, however,wherein radiation is directed with a 0° angle of incidence. Instead, ray13 (and rays 11 and 12) fall on 15 at an angle θ≧60°. As set forth inthe book Advanced Optical Techniques, Edited by A. C. S. Van Heel andbearing Library of Congress Catalog Card Number 67-20004, page 187, 15acts as a polarizer for ray 13. Rays 11 an 12, and one plane ofpolarization of ray 13 (shown as ray 13a) are thus parallel to anoptical axis between plate 15 and plate 16 (yet to be described) Theother plane of polarization of ray 13 (shown as ray 13b) is transmittedthrough plate 15, and is directed by lens 17 into absorber 18. Rays 11,12, and 13a fall on plate 16; rays 11 and 12 are reflected, and ray 13ais transmitted through 16 and lens 19 into absorber 20. All of rays 11,12, and 13a impinged onto 16 at an incident angle θ≧60°. Plate 16, whichis another thin-film Fabry-Perot interference filter, is rotated about aline (optical axis) between 15 and 16 may be visualized as follows:imagine the bottom edges of 15 and 16 resting on a common plane. Plate15 is inclined to this plane at an angle of 30° or less, such that ray13a, when reflected from plate 15, is parallel to the common plane.Plate 16 is perpendicular to the common plane and at an angle withrespect to ray 13a such that ray 13a impinges at 60° or greater, and istransmitted by 16. Rays 11 and 12 are reflected and are focussed by lens21 onto photodetector 22. For the sake of illustration, 22 is shown as ahuman eye, which it may be if the scene is in daylight and is beingobserved in visible light. For low visible-light levels, and I² or IRphotodetector may be used; IR photodetectors may also be used for ascene having a hidden (by camoflage or folige) target, as is well knownin the art.

Although no specific high-power laser has thus far been identifiedherein, one which comes to mind is the CO₂ laser operating at 10.6μm. AFabry-Perot interference filter for this wavelength cannot be simplyconstructed of silver or aluminum partial reflecting layers, but mustuse gold or some other efficient reflector for 10.6 μm. Whatever thewavelength of the laser, the most desirable Fabry-Perot filter isprobably one composed of thin films, by the well-known techniques.Obviously, since plates 15 and 16 are wavelength sensitive, they must bechosen in accordance with the wavelength of the countermeasure laserbeing used.

I claim:
 1. An optical device for separating radiation of a particularpredetermined wavelength from an optical scene emitting or reflecting aspectrum including a plurality of radiation wavelengths, the deviceincluding:first polarizing means consisting of a Fabry-Perotinterference filter for said particular predetermined wavelength; meansfor directing the scene spectrum onto said first polarizing means at anangle of incidence at least equal to 60°, whereby one plane ofpolarization of said particular predetermined wavelength is transmittedand at least a portion of the remainder of said spectrum is reflected bysaid first polarizing means; second polarizing means, consisting of aFabry-Perot interference filter for said particular predeterminedwavelength, whereby the scene spectrum reflected from said firstpolarizing means falls on said second polarizing means at an angle ofincidence at least equal to 60°, and said second polarizing meanstransmits the other plane of polarization of said particularpredetermined wavelength and reflects at least a portion of theremainder of said spectrum; a photodetector; and means for directingthat portion of said spectrum reflected by said second polarizing meansonto said photodetector.
 2. The device as set forth in claim 1 furtherincluding means for absorbing radiation transmitted by said first andsecond polarizing means.